The present invention relates to communications systems in general and, more particularly, to a method and apparatus for smoothly transmitting bursty data in a wireless communications system and a corresponding signalling method.
Existing CDMA (code-division multiple access) systems based on TIA/EIA standard IS-95 support the transmission of digital information at rates of up to 9.6 Kbps or at most 14.4 Kbps per channel. However, these data rates are considered to be insufficient for supporting many emerging data transfer applications, including browsing of the World Wide Web, electronic mail, e-commerce, tele-medicine and the like. With the goal of providing greater flexibility in the delivery of data services while continuing to exploit the features and merits of a CDMA-based system, various North American, European and Japanese consortia have developed proposals for what has become known as third generation (3G) CDMA.
As a consequence of 3G CDMA being able to support a wide range of data rates, links having different data rates may vary considerably in the power they consume. For example, to maintain the same quality of service for a 384 Kbps link, approximately 16 dB more power is required than for a 9.6 Kbps link. (The latter is typical of the bit rate of a standard voice channel in both IS-95 and 3G CDMA, while the former is typical of the bit rate that has been proposed for use by high-speed data channels in 3G CDMA.) Thus, the abrupt start of a 384 Kbps transmission is roughly equivalent to the simultaneous origination of 40 voice calls.
The interference caused by establishing high-speed data connections depends on various factors, such as the locations of the users in a cell, the direction of communication (forward-link or reverse-link) and the cell loading. By way of illustration, let there be a high-speed link established between a first mobile unit and a base station in a cell C1. If a second mobile unit is also located in cell C1 and if the first mobile unit""s high-speed connection is in the forward-link direction, then very little interference will be felt by the second mobile unit, due to mutual orthogonality of the signals transmitted by the base station in cell C1.
On the other hand, if the second mobile unit is located in a cell C2 which borders with cell C1 and if the first mobile unit""s high-speed connection has been established in the forward-link direction, then factors such as the loading of cell C2 and the proximity of the second mobile unit to the boundary of cell C1 will determine the level of interference felt by the second mobile unit. This can range from very mild (when cell C2 is heavily loaded and the second mobile unit is far away from the boundary between cells C1 and C2) to very severe (when cell C2 has very few active calls and the second mobile unit is proximate the boundary between cells C1 and C2).
An increase in the interference felt by other users (e.g., the second mobile unit in the above example) results in a degradation in the quality of service of the connections established by those other users. Consequently, the transmitted power of the affected links will be increased using standard power control algorithms until an acceptable quality of service is again attained. However, the other users may not be able to increase their power levels quickly enough, which may result in frames being lost or delayed in the meantime. Specifically, if there is an overlay of the 2G and 3G systems, the users of the 2G system will suffer loss of a greater number of frames. Fast power control is not employed in 2G systems. The 2G users will require a longer time to respond to the increase in the interference level.
Furthermore, even if the other users are capable of raising their power levels quickly, a certain amount of time will elapse before stable power levels are arrived at by the various users. This is due to the fact that when link power is increased to satisfy any one user, interference will be caused to the remaining users, who then have to raise their respective link power, thereby affecting the user who originally required an increase in link power, and so on. This chain reaction continues until all users reach a stable power level but in the meantime, some users may experience a lower signal quality than required. Depending on the degree of burstiness of the high-speed data, such chain reactions may be initiated many times per second and thus the power levels may not be given a chance to converge.
Clearly, a development problem of the 3G CDMA systems is that of addressing the sudden interference changes resulting from the bursty nature of high-speed data transmissions.
It is an object of the present invention to provide an improved method and apparatus for smoothly transmitting bursty data in a wireless communications system.
According to one aspect of the present invention, there is provided a method of smoothing bit rate transitions in a bursty input data stream arid providing an output data stream having smoother data rate transitions than the input data stream, the method comprising the steps of: receiving the input data stream in data store means; providing a rate control signal with reference to a control message containing system parameters; and withdrawing the data from the data store means at a rate determined by the rate control signal.
According to another aspect of the present invention, there is provided a transmission system comprising: data source means for providing an input data stream comprising data bursts; data store means for receiving the input data stream; control means for providing a rate control signal in responsive to a control message; and processing means for withdrawing data from the data store means at a rate indicated by the rate control signal, so as to produce an output data stream which is less bursty than the input data stream.
According to another aspect of the present invention, there is provided a wireless-link communications system comprising signal transmitter and receiver, the transmitter comprising: data source means for providing an input data stream comprising data bursts; data store means for receiving the input data stream; control means for providing a rate control signal in responsive to a control message; processing means for withdrawing data from the data store means at a rate indicated by the rate control signal, so as to produce an output data stream which is less bursty than the input data stream; and transmission means for transmitting a signal in response to the output data stream to the receiver over a wireless link, the receiver comprising: receiving means for the signal from the transmitter; reproducing means for reproducing the data from the signal received by the receiving means.
For example, the control message is sent to a receiver to communicate for power ramp-up and ramp-down profiles. The control message contains a plurality of assignments. Each assignment comprises a plurality of fields, each field having a group of parameter data. The parameter data includes rate and time period information. The rate of the rate control signal is varied in response to the rate information of the parameter data. The rate of the rate control signal is set during the time period determined by the time period information of the parameter data. The data is withdrawn from the data store means at the rate set in response to the rate control signal. Smoothing the bursty data stream reduces the degradation of service quality to other users of the system upon establishing the bursty connection. The problems related to convergence of power levels across the entire system are practically eliminated, as well.
The transmission system may comprise transmitter means for scaling the output of the processing means with reference to a variable power of the rate control signal provided by the control means. In a case where the transmission system is implemented in a wireless communications system, after a high data rate user is admitted and a supplemental channel is assigned to the user, a control message is sent to a receiver to communicate for power ramp-up and ramp-down profiles, the control message containing a plurality of assignments.
Power amplifier requirements in the transmitter means may be relaxed, which can prevent outages of the high-speed link itself. Also, since there are fewer abrupt changes in the output rate, there will be fewer abrupt changes in the induced interference, leading to a lower percentage of call drops which are retained and hence the capacity is increased.
The transmission system may be part of either a mobile unit or a base station.
For a given final data rate, transmitting only one message containing all the intermediate data rates and their duration decreases the overhead thereby increasing the overall capacity of the system. Also, the data rate changes can be done as quickly as possible to avoid unnecessary delay in high data rate transmission.